Phosphoinositide 3-kinases (PI3Ks) have a critical signalling role in cell proliferation, cell survival, vascularization, membrane trafficking, glucose transport, neurite outgrowth, membrane ruffling, superoxide production, actin reorganization and chemotaxis (Cantley, 2000, Science, 296, 1655-1657 and Vanhaesebroeck et al., 2001, Annu. Rev. Biochem., 70, 535-602).
The term PI3K is given to a family of lipid linases which, in mammals, consists in eight identified PI3Ks that are divided into three sub-families according to their structure and their substrate specificity.
Class I group of PI3Ks consists in two sub-groups, Class IA and Class IB.
Class IA consists in a 85 kDa regulatory unit (responsible for protein-protein interactions via the interaction of Src homology 2 (SH2) domain with phosphotyrosine residues of other proteins) and a catalytic sub-unit of 110 kDa. Three catalytic forms (p100α, p110β and p100δ) and five regulatory isoforms (p85α, p85β, p55γ, p55α and p50α) exist for this class.
Class IB are stimulated by G protein βγ sub-units of heterodimeric G proteins. The only characterized member of Class IB is PI3Kγ (p110γ catalytic sub-unit complexed with a 101-kDa regulatory protein, p101).
Class II PI3Ks comprises α, β and γ isoforms, which are approximately of 170 kDa and characterized by the presence of a C-terminal C2 domain.
Class III PI3Ks includes the phosphatidylinositol specific 3-kinases.
The evolutionary conserved isoforms p110α and β are ubiquitiously expressed, while δ and γ are more specifically expressed in the haematopoetic cell system, smooth muscle cells, myocytes and endothelial cells (Vanhaesebroeck et al., 1997, Trends Biochem Sci., 22(7), 267-72). Their expression might also be regulated in an inducible manner depending on the cellular-, tissue type and stimuli as well as disease context.
PI3Ks are enzymes involved in phospholipid signalling and are activated in response to a variety of extra-cellular signals such as growth factors, mitogens, integrins (cell-cell interactions) hormones, cytokines, viruses and neurotransmitters and also by intra-cellular cross regulation by other signaling molecules (cross-talk, where the original signal can activate some parallel pathways that in a second step transmitt signals to PI3Ks by intra-cellular signaling events), such as small GTPases, kinases or phosphatases for example.
Phosphatidylinositol (PtdIns) is the basic building block for the intracellular inositol lipids in eukaryotic cells, consisting of D-myo-inositol-1-phosphate (Ins1P) linked via its phosphate group to diacylglycerol. The inositol head group of PtdIns has five free hydroxy groups and three of these are found to be phosphorylated in cells in different combinations. PtdIns and its phosphorylated derivatives are collectively referred as inositol phospholipids or phosphoinositides (PIs). Eight PI species have been documented in eukaryotic cells (Vanhaesebroeck et al., 2001, above). PIs all reside in membranes and are substrates for kinases, phosphatases and lipases.
In vitro, PI3Ks phosphorylate the 3-hydroxyl group of the inositol ring in three different substrates: phosphatidylinositol (PtdIns), phosphatidylinositol-4-phosphate (PI(4)P) and phosphatidylinositol-4,5-biphosphate (PI(4,5)P2), respectively generating three lipid products, namely phosphatidylinositol 3-monophosphate (PI(3)P), phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3 (see Scheme A below).

The preferred substrate for Class I PI3Ks is PI(4,5)P2. Class II PIKs have a strong preference for PtdIns as substrate over PI(4)P and PI(4,5)P2. Class III PI3Ks can only use PtdIns as substrate in vivo and are likely to be responsible for the generation of most PI(3)P in cells (Vanhaesebroeck et al., 2001, above).
The phosphoinositides intracellular signalling pathway begins with the binding of a signalling molecule (extracellular ligands, stimuli, receptor dimerization, transactivation by heterologous receptor (e.g. receptor tyrosine kinase)) to a G-protein linked transmembrane receptor integrated into the plasma membrane resulting in the activation of PI3Ks.
Once activated, PI3Ks convert the membrane phospholipid PI(4,5)P2 into PI(3,4,5)P3 which in turn can be further converted into another 3′ phosphorylated form of phosphoinositides by 5′-specific phosphoinositide phosphatases, thus PI3K enzymatic activity results either directly or indirectly in the generation of two 3′-phosphoinositide sub-types that function as second messengers in intra-cellular signal transduction (Leslie et al., 2001, Chem. Rev. 101(8) 2365-80; Katso et al, 2001, Annu. Rev. Cell Dev. Biol. 1, 615-75 and Toker et al, 2002, Cell Mol. Life. Sci. 59(5) 761-79).
The role as second messengers of phosphorylated products of PtdIns act is involved in a variety of signal transduction pathways, including those essential to cell proliferation, cell differentiation, cell growth, cell size, cell survival, apoptosis, adhesion, cell motility, cell migration, chemotaxis, invasion, cytoskeletal rearrangement, cell shape changes, vesicle trafficking and metabolic pathway (Stein, 2000, Mol. Med. Today 6(9) 347-57). Chemotaxis—the directed movement of cells toward a concentration gradient of chemical attractants, also called chemokines is involved in many important diseases such as inflammation/auto-immunity, neurodegeneration, angiogenesis, invasion/metastasis and wound healing (Wyman et al., 2000, Immunol Today 21(6) 260-4; Hirsch et al., 2000, Science 287(5455) 1049-53; Hirsch et al, 2001, FASEB J. 15(11) 2019-21 and Gerard et al., 2001, Nat Immunol 2(2) 108-15).
PI3-kinase activation, is therefore believed to be involved in a range of cellular responses including cell growth, differentiation and apoptosis (Parker et al., 1995, Current Biology, 5, 577-99; Yao et al., 1995, Science, 267, 2003-05).
Recent biochemical studies revealed that, Class I PI3Ks (e.g. Class IB isoform PI3Kγ) are dual-specific kinase enzymes, i.e. they display both lipid kinase activity (phosphorylation of phospho-inositides) as well as protein kinase activity, as they are able to induce the phosphorylation of other protein as substrates, including auto-phosphorylation as intra-molecular regulatory mechanism.
PI3Ks appear to be involved in a number of aspects of leukocyte activation. A p85 associated PI3-kinase activity has been shown to physically associate with the cytoplasmic domain of CD28, which is an important co-stimulatory molecule for the activation of T-cells in response to antigen (Pages et al., 1994, Nature, 369, 327-29). These effects are linked to increases in the transcription of a number of genes including interleukin-2 (IL2), an important T cell growth factor (Fraser et al, 1991, Science, 251, 313-16). Mutation of CD28 such that it can longer interact with PI3-kinase leads to a failure to initiate IL2 production, suggesting a critical role for PI3-kinase in T cell activation.
Cellular processes in which PI3Ks play an essential role include suppression of apoptosis, reorganization of the actin skeleton, cardiac myocyte growth, glycogen synthase stimulation by insulin, TNFα-mediated neutrophil priming and superoxide generation, and leukocyte migration and adhesion to endothelial cells.
PI3Kγ has been identified as a mediator of G beta-gamma-dependent regulation of JNK activity wherein G beta-gamma are subunits of heterotrimeric G proteins (Lopez-Ilasaca et al., 1998, J. Biol. Chem. 273(5) 2505-8).
Recently, it has been described that PI3Kγ relays inflammatory signals through various G(i)-coupled receptors (Laffargue et al, 2002, Immunity 16(3) 441-51) and its central to mast cell function, stimuli in context of leukocytes, immunology includes cytokines, chemokines, adenosines, antibodies, integrins, aggregation factors, growth factors, viruses or hormones for example (Lawlor et al., 2001, J. Cell. Sci., 114 (Pt 16) 2903-1 and Stephens et al., 2002, Curr. Opinion Cell Biol. 14(2), 203-13).
Specific inhibitors against individual members of a family of enzymes provide valuable tools for deciphering functions of each enzyme.
Two compounds, LY294002 and wortmannin (cf. hereinafter), have been widely used as PI3-kinase inhibitors. These compounds are non-specific PI3K inhibitors, as they do not distinguish among the four members of Class I PI3-kinases.

IC50 values of wortmannin against each of the various Class I PI3-kinases are in the range of 1-10 nM and IC50 values for LY294002 against each of these PI3-kinases are about 15-20 μM (Fruman et al., 1998, Ann. Rev. Biochem., 67, 481-507), also 5-10 mM on CK2 protein kinase and some inhibitory activity on phospholipases.
Wortmannin is a fungal metabolite which irreversibly inhibits PI3K activity by binding covalently to the catalytic domain of this enzyme. Inhibition of PI3K activity by wortmannin eliminates the subsequent cellular response to the extracellular factor (Thelen et al., 1994, Proc. Natl. Acad. Sci. USA, 91, 4960-64). Experiments with wortmannin, show that PI3K activity in cells of hematopoietic lineage, particularly neutrophils, monocytes, and other types of leukocytes, is involved in many of the non-memory immune response associated with acute and chronic inflammation.
Based on studies using wortmannin, there is evidence that PI3-kinase function is also required for some aspects of leukocyte signaling through G-protein coupled receptors (Thelen et al., 1994). Moreover, it has been shown that wortmannin and LY294002 block neutrophil migration and superoxide release. However, in as much as these compounds do not distinguish among the various isoforms of PI3K, it remains unclear which particular PI3K isoform or isoforms are involved in these phenomena.
Some results have indicated that PI3K inhibitors, for example, LY294002, can increase the in vivo antitumor activity of certain cytotoxic agents (e.g. paclitaxel) (Grant, 2003, Current Drugs, 6(10), 946-948).
Recently, 5-phenylthiazole derivatives have been recently developed as PI3K inhibitors (WO 03/072557).
The high relevance of the PI3K pathway in some widely spread diseases stresses the need to develop inhibitors, including selective inhibitors, of PIKs.